Monday, March 20, 2006

What's the mileage on that bicycle?

"Huh! what's that again?""You heard that right. Do you know the mileage on that bicycle?"

The logic goes thus. Everything we eat is produced somewhere, processed somewhere and transported to our supermarket and from there to our homes. It takes energy to do that. A major contribution of the energy needed to produce, process and transport our food comes from fossil fuels (the stuff USA is officially addicted to, since January'06). When you eat that sandwich, you are effectively gulping down gasoline. Every time you ride that bicycle, you are burning some of the same energy which came from the fossil-fuels. Naturally this bears the question, "How many miles can you go on that bicycle for every gallon of gasoline you consume (indirectly... through the food you eat)?". Got it? Good!

Lets go over the answer step by step:1> How much energy does a gallon of gasoline have?This is called the calorific value of the fuel. This EPA webpage says that the energy content of gasoline is an average 113,000 BTU/gallon. That translates to 28,476 kcal/gallon. (1 BTU = 0.252 kcal)

2>How many gallons of gasoline do you (indirectly) consume?This depends on the type of your diet. The table below shows the amount of fossil fuel input for different types of diet. These numbers are from the book, "Food energy and society", by David and Marcia Pimentel [1]. Note that all these numbers are for a 3600 kcal daily diet (remember 1 food Cal. = 1 kcal). It is interesting to note here that average American eats ~1500 Cal. more than the daily requirement of 2100 Cal. I have converted the fossil fuel input number into gallons of fuel using the calorific value given above.

3> How many food Calories do we burn while bicycling?My search yielded a number of different values. I found this table on howstuffworks.com and a few other websites [this pdf, page 21]. It says that 0.049 Calories are burnt per pound, per minute while cycling at the speed of 15 mph. Another table is given in the Dietary guidelines for America, and can be found on many websites [CDC webpage, also this pdf - page 4]. This says that a person weighing 154 lb, will burn 290 Cal. when bicycling at a speed less than 10 mph, for one hour. Translates to 0.031 Cal. per lb per minute. A person weighing 154 lb, bicycling at 10 mph will have to go 125.7 mile to burn 36oo Cal. daily intake.

27 comments:

If you talk about indirect mileage, shouldn't you also take into account the amount of gasoline spent in the process of manufacturing and transporting the bicycle. You can probably divide it by the lifecylce of bicycle to get gasoline used per mile

shouldn't you also take into account the amount of gasoline spent in the process of manufacturing and transporting the bicycle.

Not really. The intention is to compare with the mileage as measured for cars. The only fuel input needed to "drive" the bicycle is (indirectly) thru food. In case of car we measure miles per gallon of fuel needed to "drive" the car.

Actually we also probably burn some food calories in driving the car but my guess was that isn't a lot and will not account for any mileage decrease for the car. Drinking coke/coffee while driving means we may gain food calories while driving too :-)

As a measure we can indeed compare cost of energy input in the production process per unit weight of the bicycle with that for a car perhaps. I dunno what it will look like.

@Anonymous : You are correct. Local food is much better than food transported thousands of miles away. The "food miles" component is indeed pretty large. When it comes the food choices opting for Local, Organic, Fresh/Non-processed (in that order) seems most reasonable.

I had linked the article "eating fossil fuels" in one of my previous posts. It has a good summary of the food miles portion in US diet.

I'm not an expert on this stuff, but I think you do indeed need to take into account the Basal metabolic rate, more or less the fancy name for you normal caloric burn. For most people the BSM is about 1900 calories a day. So a 15 mile commute on a bicycle would be 2 hours a day of commuting (at 15mph). The normal burn during that time would be something like this: (1900cal/24hrs)*2hrs commuting = 158 calories. That's 158 calories you'd burn anyway just sitting in the car or being alive in general. That's significant given the numbers involved, so you'd have to add that to car's energy use or subtract it from the bike's for a fair comparison. So, the bike is even more efficient.

I think it makes more sense to estimate the mass of food required and then project the amount of gasoline required to transport that mass of food. Your current relationship seems to me to imply that I'm directly consuming metabolizing the gasoline. The kcal in the gas is irrelevant.

1500 miles per gallon @ $3 per gallon of gasoline vs 20 mile charge cost 5 cents.Electric bikes typically travel at speeds of 10 to 20+ miles per hour and go up to 12-20 miles charge. Charging your electric bike for four to six hours uses 5 cents worth of electricity from common 110 wall outlets. The electric bike power can give you a break from pedaling or supply more power when you're going up hill. http://www.bicycles-electric-bikes.com/bike_to_work.htm

Americans love nature and the out of doors, it is part of our national heritage. Many people have come to experience nature out the window of a car, while sliding down a ski slope or on TV. Given the opportunity to enjoy the out of doors on a silent, affordable urban transit enthusiasts will choose to do so. The electric bicycle is silent, does not stink or vibrate, is inexpensive and reliable.

Electric Bikes appeal to all recreational riders, and many other market segments. Seniors want to get outdoors and get exercise, but are not always able to pedal themselves up a hill. Historically fitness and outdoors oriented, will turn to electric bicycles as a way to extend their present habits and preferences. Commuters frustrated with traffic congestion, and rising costs of car ownership, find electric bicycles a very practical transportation choice. When used as a power assisted bike, the rider's input can be enhanced, and can continue over a longer period of time than on a regular bike.

@anonymous1: I am taking into account the fossil fuel required to "grow, process and transport food" NOT only to transport food.

Regarding the way the calculation is set up it doesn't matter which way you start you should get similar answers. In this case the kcal in gasoline matter because the Pimentel study gives the fossil fuel input for each type of diet in kcal and not in gallon. I need to convert it to gallons to calculate "miles per gallon".

regarding "gulping gasoline", if you take a look at how much of the food distribution/production is dependent on gasoline, "gulping gasoline" is not very far from the truth.

you use a pedaling speed of 'less than 10mph'. This is a pretty slow pace for any kind of commute. I'd think a fairer number would be 15mph, where the human would be using more kcals (possibly more than 50% more).

As a physicist I find your math fatally wrong:Using your figure: 125 miles per 3,600 Cal implies 0.03 miles per Cal. There are 28,000 Cal in 1 gal of gasoline. So: 0.03 mile/Cal times 28,000 Cal/gal equals 970 miles per gal.

So the human body powering a bike could go a thousand miles for the same amount of energy stored in a gal of gasoline. It is more than 30 times more efficient than an average car.

It is interesting to note that the highest mileage car (or something like a car) on record is actually ten times more efficient still: on the order of 10,000 miles / gal !

There are 28,000 Cal in 1 gal of gasoline.So:0.03 mile/Cal times 28,000 Cal/gal equals 970 miles per gal.Your calculation (970 miles/gallon) would be correct in the hypothetical world in which humans "directly" consume gasoline as the source of their energy. Thus requiring only ~0.126 gal. of gas for the daily 3600 kcal consumed.

In the real world we do consume gasoline "indirectly". Every K.Cal of (food) energy intake requires a certain gallons of gasoline input. That's what I have tried to investigate in point #2 in the post. It turns out that we need a lot more than 0.126 gal. of gasoline input to get our 3600 kcal daily intake. With the average American diet it takes about 10 times as much gasoline intake to get (i.e. produce, pack, transport etc.) the 3600 kcal equivalent of food.

That is the reason why the numbers are what they are. I really doubt that the math is wrong here. I guess I just have a bit of a trouble in explaining the logic.

I do apologize for not reading your post throughly. I now see what you were investigating.So the revelations are that , using ref. 1's figures, in terms of fossil fuel use an average diet is 2 times less efficient than a vegetarian diet and that (base on my figure) we could be 5 to10 times more efficient still. This brings up the question of why there is a reduction in efficiency in the first place. That is, if no fossil fuel is use in our food caloric production, if all people eat out of their own farm, say, and all people only bike or walk all the time, we would have the fossil-fuel-to-calorie ratio for transportation of zero. But we would have reverted back to the age of pre-industrial revolution. So we need to ask what have fossil fuel allow us to do that we can't do 300 years ago and was it all worth it. The answers come in the form of our increased access to speed, power, and convenient. Our ability to extract and use vast amount of easily accessible (but dirty) sources of energy has made us addicted not to the energy sources themselves, but to the way of life they allow us to live. I would guess that few people today, even the most fervent environmentally friendly ones, would totally replace air travel with bike riding. We expect to build a skyscraper in years and can no longer tolerate hand-built, centuries-long cathedral projects. Millions of people can not exist if fossil fuel is not available to drive food transport. We can't un-invent plastic. We do not have an addiction but a hard-to-sustain life style, and that, unlike an addiction, has no easy cure. It is not the "fault" of fossil fuel that we didn't have the foresight to predict all its impacts back when we decided to exploit it --but that's just being human for you.Sorry to digress the thread....back to thinking about mpg.

It's an interesting mental exercise, but I tend to approximate the carbon footprint of bicycling at 0 since (I hope) I'd exercise the same number of calories away for health reasons even if I weren't doing it for transportation.

But what is the true caloric value of a gallon of gasoline then? This takes into account the energy required to bring me the food but NOT the energy required to put the gasoline in my tank. In that case, just comparing the food energy to the gasoline energy would be more accurate.(fantastic article by the way)

You should also account for the fact that you shower more often and you wash your clothes more often when you commute by bike versus a car. Generating the extra water and heating it up needs fossil fuels...

Clearly the 10 October Anonymous poster isn't a bike commuter! Shower more often? I shower after I commute - once per day. The only thing that changes is when during the day that shower occurs. As for washing clothing, most bike commuting garments are synthetic and very quick drying. I seldom do more than rinse with cold water to remove salt, then wash at the end of the week. Since these garments are less bulky, perhaps there's energy savings as I can fit more in my washer.

Can you say reductio ad absurdum?

Fact: I have a car (honda hybrid)Fact: I have a bikeFact: When I commute to work on my bike (45 mile round trip), I'm producing less carbon than when I drive.

This is an excellent blog for electric bicycles. There are not too many around like this. Thanks for making this such an interesting subject. Oh, by the way, Wired Magazine has a great article on hybrid cars this month. (Jan 2008 issue).

I just heard a report on the radio that either there was going to be a very high tax or a ban completely on electric scooters and bicycles. I got so damn mad I thought my head was going to break open. The reason for the ban was because of studies showing that the batteries on these vehicles were too volatile and were prone to explosions.

I think they are initially going to say there is a ban when there really isn't one, so that the tax on them won't seem so bad.

Anyway, I wrote an article on the story on my blog at www.electricbikesnscooters.com

Excellent blog and conversation. I have wondered about this issue myself. However, what I really want to do is poke my finger in the eye of all those Prius owners by saying my "vehicle" (ie: bike) gets a hell of a lot better mileage than their "green" car. What MPG can I through back at them if push comes to shove?

Actually we also probably burn some food calories in driving the car but my guess was that isn't a lot and will not account for any mileage decrease for the car. Drinking coke/coffee while driving means we may gain food calories while driving too